Assessment of antimicrobial resistance in pathogens responsible for causing bovine mastitis in Kentucky dairy farms

Clinical and subclinical bovine mastitis is of concern in the global dairy industry. The most predominant causative agents of bovine mastitis are from the streptococci family, including Streptococcus dysgalactiae, S. uberis, and S. agalactiae. Staphylococcus aureus was historically considered to be the predominant causative agent among staphylococci. However, recent research has shown that the coagulase-negative staphylococci (CNS) are an increasingly important group in bovine mastitis. Antibiotic resistant bacteria continue to be a pervasive problem; therefore, identifying specific trends in resistant bacteria can lead to faster treatment and reduced monetary loss. In this study we assessed the presence of antimicrobial-resistant bacterial strains responsible for mastitis in Kentucky dairy cows. From 2011-2012, 308 milk samples were obtained from cows selected based on their recent somatic cell count (SCC). Isolated species were identified using the Gram stain technique and characterized by biochemical testing. Isolates were tested using 11 common antibiotics via the Kirby Bauer test. Resistance was highest in streptococci, with isolates resistant to kanamycin (69%), tetracycline (17%), and oxacillin (50%). Matrix assisted laser desorption ionization time of flight (MALDI-TOF) was used to confirm biochemical testing of identified resistant strains and to identify CNS resistant strains to the species level. Tetracycline resistant streptococcal species were tested for tetO, tetS and tetM genes using polymerase chain reaction (PCR). 21% indicated the presence of tetO gene, 14% tetM and 28% tetS